Exemplary embodiments of the present invention relate to a method and a device for generating a data stream comprising user messages in a satellite-based augmentation system.
In order to increase the accuracy of position determination with a global navigation satellite system (GNSS: Global Navigation Satellite System) such as (NAVSTAR-) GPS, GLONASS or the future European navigation satellite system GALILEO, satellite-based augmentation systems (SBAS: Satellite Based Augmentation System) such as the European EGNOS (European Geostationary Navigation Overlay Service), the US-American WAAS (Wide Area Augmentation System) or the Japanese MSAS (Multifunctional Satellite Augmentation System) are used.
An SBAS uses additional satellites for a GNSS to be augmented, which satellites transmit regionally limited correction data that can be used by suitable receiver systems (SBAS receiver) of the GNSS for improving the accuracy of position determination. An augmentation system can also transmit integrity data or integrity information to the augmented GNSS, which allows the receivers to determine an integrity risk and to alert in the case of a risk that is too high. For example, besides correction data, EGNOS transmits also data on the integrity of GPS.
EGNOS comprises a network of 34 RIMS (Ranging and Integrity Monitoring Station), which receive navigation signals of the augmented GNSS, currently GPS and GLONASS. Correction and integrity data (EGNOS data) data are calculated from the data of the RIMS in four redundant MCCs (Master Control Center). The EGNOS data calculated by the MCC are transferred by six uplink stations NLES (Navigation Land Earth Station) in the form of GEO messages to the three geostationary EGNOS satellites (GEO satellites), which then distribute the EGNOS data in the form of a data stream comprising user messages to receiver systems. In addition, the EGNOS data are also made available through the Internet so that in the case of disturbed EGNOS satellite reception, receiver systems with Internet access still can receive current EGNOS data.
Calculating the EGNOS data in an MCC is carried out by a CPF (Central Processing Facility). The EGNOS message, which comprises EGNOS data, is generated by an MGF (Message Generation Facility) in the MCC and is provided for the transmission through the uplink stations NLES. Thus, each MCC generates with its CPF and MGF its own EGNOS messages which are available to the NLESs for transmission. Different types of EGNOS messages are defined. For example, EGNOS messages of the types 2-5 transmit corrections of the individual satellites of the augmented GNSS, and EGNOS messages of the type 6 transmit integrity information.
Each uplink station NLES selects from the different CPFs one particular CPF as a data source from which the uplink station takes the generated and provided EGNOS messages and transmits them to the EGNOS satellite controlled by said uplink station. The CPF selected by an NLES is designated as “selected” and the remaining CPFs are designated as “backup”. A CPF designated as “selected” by an NLES transmits to the NLES a complete set of messages with all types of EGNOS messages, whereas the CPFs designated as “backup” transmit only integrity data (EGNOS message type 6) to the NLES. An NLES selects a certain CPF on the basis of a quality-of-service index and go/no-go flags provided by the CPF, which are transmitted separately to all NLESs by each CPF.
According to the current concept of EGNOS V2, it is not possible for a CPF and also for individual MGFs to determine via a data stream comprising EGNOS user messages which MGF and thus also which CPF has been selected by an NLES for generating GEO messages. For example, if one of the three EGNOS satellites receives a data stream comprising EGNOS user messages, it is not possible to determine on the basis of the EGNOS user messages which CPFs, and thus also MGFs, the NLES, which provides data to the EGNOS satellite, has selected as data sources from all CPFs and MGFs present in EGNOS.
Exemplary embodiments of the present invention provide a method and device for generating a data stream comprising user messages in a satellite-based augmentation system, wherein the data stream is suitable for identifying a data source used for generating user messages of a satellite-based augmentation system, in particular for identifying a CPF and an MGF selected by an NLES as a source for EGNOS user messages.
In accordance with exemplary embodiments of the present invention, a satellite-based augmentation system the data stream comprising user messages is encoded so as to indicate the augmentation system's data source used for generating the user messages. Here, encoding shall take place in such a manner that no additional interfaces have to be introduced. As a suitable encoding type, the invention proposes the sequence of different message types of the user messages in a data stream. As already explained above, in the case of the EGNOS, for example, corrections of the individual satellites of the augmented GNSS are transmitted with messages of the types 2-5. By allocating in each case a unique sequence of message types to data sources in the augmentation system, it is therefore possible that the data streams comprising user messages from the data sources can be configured differently so that identifying the data source used for a particular data stream comprising user messages is made possible. For example, in the case of EGNOS, different sequences of the message types 2-5 can be allocated to the data sources CPF and MGF available in EGNOS so that the CPF and the MGF, which were selected for generating the EGNOS user messages by the NLES which provides data to the EGNOS satellite, can be identified on the basis of the data stream comprising EGNOS user messages transmitted by an EGNOS satellite. Thus, each MGF and hence also each CPF can determine, based on a data stream comprising EGNOS, user messages which CPF and MGF were selected by an NLES for the respective EGNOS satellite.
One embodiment of the invention relates to a method for generating a data stream comprising user messages in a satellite-based augmentation system, wherein the user messages can exhibit different message types, and the method comprises the following steps:
Forming a plurality of predetermined unique sequences of message types and allocating each formed sequence to in each case one of a plurality of data sources that are used for generating user messages in the satellite-based augmentation system,
Selecting a data source for generating a data stream comprising user messages, and
Generating, by the selected data source, the data stream comprising user messages by using the predetermined unique sequence of message types that is allocated to the selected data source.
In particular, for forming unique sequences, such message types are used that have to be contained in each data stream comprising user messages.
In particular, message types provided for transmitting fast corrections of satellites of an augmented global navigation satellite system can be used as message types.
The number of the message types used can be selected based on the number of data sources that is allocated a unique sequence of message types.
The method is in particular suitable for the use in the augmentation system EGNOS V3 phase A.
Another embodiment of the invention relates to a computer program comprising a program code for carrying out all method steps according to the invention and as described herein if the computer program is executed in a computer.
Furthermore, one embodiment relates to a data carrier on which is stored the program code of the computer program according to the invention and as described above, which program code is executable by a computer.
Another embodiment of the invention relates to a device for generating a data stream comprising user messages in a satellite-based augmentation system, wherein the user messages can comprise different message types, and the device comprises the following:                first means for forming a plurality of predetermined unique sequences of message types and for allocating each formed sequence to in each case one of a plurality of data sources that are used for generating user messages in the satellite-based augmentation system,        second means for selecting one of the data sources for generating a data stream comprising user messages, and        third means for generating, by the selected data source, the data stream comprising user messages by using the predetermined unique sequence of message types that is allocated to the selected data source.        
The device can be designed for carrying out a method according to the invention and as described herein.
Finally, one embodiment of the invention relates to a receiver for a data stream comprising user messages of a satellite-based augmentation system, which receiver is produced according to a method according to the invention and/or by a device according to the invention, wherein the receiver is designed so as to recognize in the received data stream a unique sequence of message types of user messages and to identify on the basis of the recognized unique sequence a data source of user messages.
Further advantages and possible uses of the present invention arise from the following description in connection with the exemplary embodiments illustrated in the drawings.
In the description, the claims, the abstract and in the drawings, the terms and associated reference numbers in the reference list below are used.